Hot swapping refers to replacing an electrical (or a computer) system's component without shutting down the electrical system. In conventional systems, hot swapping is managed by hot swap controllers. Hot swap controllers can be implemented on integrated circuits (ICs) and used in applications such as server boards and power supplies to manage the hot swapping of components such as batteries. Conventional hot swap controllers use one or more paralleled metal-oxide-semiconductor field-effect transistors (MOSFETs), through which the current drawn by the newly added load flows.
If a hot swap controller uses only one MOSFET, that MOSFET would need to support 100% of the load current. However, conventional hot swap controllers often use multiple MOSFETs coupled in parallel, so that the load current is divided between the MOSFETs and the conduction losses are reduced. When a new device or load is initially plugged into an electrical system, the new load's uncharged power supply filter capacitors present low impedance and demand a large and sudden “inrush” current. Inrush currents can be an order of magnitude larger than maximum steady state currents. Large inrush currents can damage electrical components and/or cause operational faults.
So, conventionally, when a new load is first plugged in to an electrical system's power source, the hot swap controller gradually decreases the on-resistance of the MOSFET(s) to limit the inrush current. The hot swap controller controls the on-resistance of the MOSFET(s) by adjusting gate voltage(s) of the MOSFET(s). That means that the MOSFETs are put in a linear mode of operation. An issue with using parallel MOSFETs in a linear mode of operation is that the MOSFETs experience substantial thermal stress and thus cannot reliably share currents equally. Specifically, during the linear mode operation, gate voltages of the MOSFETs are not set to allow for maximum possible current flow and so the currents flowing through the MOSFETs can vary depending on gate voltage level.
Thus, to maintain a reliable hot swap control, MOSFETs with a maximum rating (e.g., each MOSFET supporting 100% load current) are typically required to withstand high, varying and unpredictable inrush currents, even though the MOSFETs are coupled to each other in parallel. MOSFETs that support higher current values are generally more expensive, leading to higher overall system costs. In addition, conventional hot swap control systems introduce a significant voltage disturbance in the output of DC power source due to rapid current variation, which can reset a load.